As described in U.S. Pat. No. 3,763,658, systems for the liquification of natural gas using a multicomponent or mixed refrigerant are currently in use throughout the world. Such systems typically employ a four-component refrigerant comprising nitrogen, methane, ethane, and propane which is circulated through a multizone heat exchanger in order to cool a feed stream of natural gas to the low temperatures at which it condenses to form LNG (typically -260.degree. F). In order to adequately cool feed streams of varying composition, temperature, and pressure, controls are required for varying the flow of refrigerant through the heat exchanger, the composition of the mixed refrigerant, the degree of compression applied to the mixed refrigerant, and other physical parameters effecting the operation of the main exchanger and refrigeration loop.
In a typical operating installation which employs a multicomponent refrigerant system, the overall facility is designed in accordance with certain design specifications which are intended to insure operation of the plant within predefined limits. On the basis of customer specifications of feed stream compositions and conditions, plant designers typically determine an optimum operating state for the system including compositions, temperatures, and pressures for the various parts of the mixed refrigerant loop. It has been found, however, that achieving and maintaining these design conditions are exceedingly difficult. Furthermore, variations in plant condition including feed stream composition variations, environmental variations, and defects such as leaks in compressor seals, valves and pipe joints all contribute to instability of the facility. For these reasons, typical mixed refrigerant plants operate at less than optimum efficiency. Because human operators are incapable of closely monitoring and adjusting for all of the variations inherent in an operating facility, and because of the many relationships which are not apparent even to highly skilled and experienced operators, overall plant efficiency is degraded, thus increasing the cost of plant product to the consumer.
Finally, when it is desirable to operate the LNG plant so as to attain maximal production, similar variability comes into play. Operation of the plant at maximum production inherently means less than optimum efficiency level is achieved. However, balancing production against efficiency requires degrees of control not presently attainable.